To extend life of mechanical parts used in abrasive environment such as friction, fatigue, corrosion or erosion, methods of curing the surface of the parts or coating them with wear-resistant materials have been used.
As the coating materials to improve wear resistance, materials having high hardness, that is, ceramic materials such as oxides, for example, alumina, carbides, for example, SiC or TiC, and nitrides for example, Si3N4, TiN are mostly used.
As the typical mechanical parts with wear-resistant coating structure, car engine blocks and related parts can be mentioned and particularly, to suppress the abrasion of the inner walls of cylinder bores, abundant technologies have been developed. For example, there can be mentioned Korean Patent Laid-Open No. 1997-0045010, 1998-017171, and 2003-0095739. Particularly, Korean Patent Laid-Open No. 1997-0045010 discloses a method of forming a coating membrane instead of prior cast iron liners on the inner walls of cylinder bores and in this method, wear resistance is improved by forming coating powders comprising ceramics and their mixtures on the inner walls of bores by spray pyrolysis using plasma or arc as heat source.
Korean Patent Laid-Open No. 1998-017171 discloses a method of forming wear-resistant coating layer on the bore side of aluminum cylinder blocks by plasma spray pyrolysis using silicone carbide particles.
Korean Patent Laid-Open No. 2003-0095739 discloses a method of forming a coating membrane by spraying a powder composition for spray coating on the inner walls of stainless cylinder bores while fusing it with heat source of high temperature, and the powder composition for spray coating is a mixture of alumina and zirconia.
As described above, numerous attempts to form a wear-resistant coating on metal base materials with excellent wear-resistant ceramic materials have been conducted, but all of these methods are mainly based on plasma or electric arc spray pyrolysis. Such spray methods provide base materials with powder particles by heating the powder particles to be coated nearly around or above fusion point thereby fusing at least a part thereof.
Therefore, as the ceramic particles to be coated onto base materials are heated to high temperature around 1000° C., which is a fusion temperature of a normal ceramic particle, and then provided to base materials by contact, they cause damages by heat shock to the base materials during the coating process, induce residual stress during the cooling process thereby decreasing adhesion and shorten the life of the parts.
Also, due to the particle spray of high temperature, there is an increased risk with the handling of spray machine and complicated operation line is inevitable. In addition, the fused particles of high temperature might react with impurities on metal matrix or its surface and form another compounds, thereby adversely affecting the properties of the material.
Meanwhile, periodical stress occurs by periodical cycling and thus, reciprocating machines and related parts receive continuously, repeatedly cycling stress of numerous times by engine rotation when engine works and as a result, periodical stress gives rise to fatigue-crack in the related parts of thermal engines together with localized heating in the parts and ultimately shortens the life of the parts. For instance, insert grooves which are used to insert glow plugs are formed around cylinder grooves in diesel engine blocks, wherein the region between the insert grooves and cylinder grooves has a high possibility of destruction by fatigue crack due to shortened interval and high temperature.
Hence, in many occasions, it is required that parts for use in thermal engines such as reciprocating engines and gas turbines have good resistance against fatigue crack as well as wear resistance. However, in most prior coating technologies, ceramic was coated alone. In such cases, heat transfer to metal matrix is not properly performed and high temperature is maintained. As a result, crack occurrence due to fatigue is increased and resistance against fatigue is thus decreased although wear resistance might be improved.